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  • 1
    Electronic Resource
    Electronic Resource
    Spatial Structure of Solar Coronal Magnetic Loops Revealed by Transient Microwave Brightenings (1998)
    Springer
    Solar physics 180 (1998), S. 285-298 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present the measurement of magnetic field gradient in magnetic loops in the solar corona, based on the multi-wavelength Very Large Array observations of two transient microwave brightenings (TMBs) in the solar active region 7135. The events were observed at 2 cm (spatial resolution ∼ 2=) and 3.6 cm (spatial resolution ∼ 3=) with a temporal resolution of 3.3 s in a time-sharing mode. Soft X-ray data (spatial resolution ∼ 2.5=) were available from the Soft X-ray Telescope on board the Yohkoh satellite. The three-dimensional structure of simple magnetic loops, where the transient brightenings occurred, were traced out by these observations. The 2-cm and 3.6-cm sources were very compact, located near the footpoint of the magnetic loops seen in the X-ray images. For the two events reported in this paper, the projected angular separation between the centroids of 2 and 3.6-cm sources is about 2.3= and 3.1=, respectively. We interpret that the 2 and 3.6-cm sources come from thermal gyro-resonance emission. The 2-cm emission is at the 3rd harmonic originating from the gyro-resonance layer where the magnetic field is 1800 G. The 3.6-cm emission is at the 2nd harmonic, originating from the gyro-resonance layer with a magnetic field of 1500 G. The estimated magnetic field gradient near the footpoint of the magnetic loop is about 0.09 G km=1 and 0.12 G km=1 for the two events. These values are smaller than those observed in the photosphere and chromosphere by at least a factor of 2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005020207482
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  • 2
    Electronic Resource
    Electronic Resource
    The pre-onset morphology of the 5 September 1973 flare (1978)
    Springer
    Solar physics 60 (1978), S. 323-339 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Simultaneous visible, EUV, and X-ray observations of magnetic structures before and during the onset of the flare of 5 September 1973 are co-registered and interpreted. Ninety minutes before the flare, intense EUV knots fluctuate near the loops which subsequently flare. The pre-flare loop is observed in O IV λ554, but not in X-rays, which show instead a parallel structure which is related either to a darkening filament or the subsequent flare kernels. As the full disk X-ray emission increases, first the EUV flare loop appears, then X-ray kernels form at the feet of two EUV loops, one of which overlies the activated filament. The flaring, at any given time, is confined to a single loop (or bundle of loops) whose long axis (barely) crosses the neutral line. As time progresses, the flaring moves to other (probably higher) loops sharing the off-band Hα footpoints but whose axes are rotated relative to the earlier loops by angles of about 30°. Previous interpretations of single-telescope observations are revised in this joint investigation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00156533
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  • 3
    Electronic Resource
    Electronic Resource
    Network to cell contrast at microwaves (1983)
    Springer
    Solar physics 85 (1983), S. 237-242 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using different models deduced from EUV lines for the cell and for various network components, the corresponding radio brightness temperature in the millimetric and centimetric range of wavelengths are computed. The contrast C = [T b (network)]/[T b (cell)] and the difference ΔT = T b (network) - T b (cell) are compared with the few available observations of the quiet Sun inhomogeneities performed with sufficient angular resolution. The comparison shows a satisfactory agreement with most of the observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148652
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  • 4
    Electronic Resource
    Electronic Resource
    Hydrodynamic simulations of flare/surge events (1979)
    Springer
    Solar physics 63 (1979), S. 187-200 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A one-dimensional, hydrodynamic, time-dependent model has been developed which simulates the major observed dynamics of flare associated surges. In particular, the thermodynamics, the surge mass, the time scales, the physical dimensions and the velocities of typical surge events are reproduced. The surge is created by a sudden increase in pressure at the top of the chromosphere. This pressure pulse produces a disturbance which is followed, with a time-dependent numerical solution, as it propagates upward through the transition region and into the corona. The leading edge of the disturbance is a weak shock which has only a slight effect on the original transition region and coronal thermodynamics. The major effect occurs in a region behind the shock where the temperature is decreased and the density is increased. This cool, dense region (the simulated surge event) moves upward initially and then begins falling back downward. After the material begins returning to the chromosphere, a second shock is formed in the chromosphere which propagates upward, brings the infalling material to rest and returns the atmosphere to hydrostatic equilibrium.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00155709
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  • 5
    Electronic Resource
    Electronic Resource
    Thermal and nonthermal phenomena in solar flare loops at 20 cm wavelength and in X-rays (1983)
    Springer
    Solar physics 83 (1983), S. 3-14 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present X-ray images from the P78-1 satellite for a long-lasting burst at 20 cm wavelength mapped with the Very Large Array on 19 May, 1979 by Velusamy and Kundu (1981). The decimeter wave observations were originally interpreted in terms of two models, one invoking thermal electrons radiating at low harmonics of the gyrofrequency, and the other invoking mildly relativistic electrons emitting gyrosynchrotron radiation. If indeed the 20 cm source is thermal, it should also be visible in soft X-rays, while if it is nonthermal, the soft X-ray emission should be weak or spatially or temporally distinct from the 20 cm burst. We find that only one of the three 20 cm sources was approximately co-spatial with the soft X-ray source, and that it was only partially thermal. The 20 cm burst is therefore primarily decimeter type IV emission from mildly relativistic electrons of the post-flare phase. The long lifetime (≳ 2h) and smooth temporal variation of the burst belie its nonthermal nature and suggest continuous acceleration as well as long term storage of energetic electrons.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148239
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  • 6
    Electronic Resource
    Electronic Resource
    Large-scale structure of the sun's corona from radio observations using the Clark Lake Radioheliograph (1987)
    Springer
    Solar physics 108 (1987), S. 113-129 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present meterwave maps of the solar corona made with the Clark Lake Radioheliograph at 30.9, 50, and 73.8 MHz for one solar rotation. We compare and contrast them with optical data: 10830 Å maps, white-light coronagraph images (SOLWIND and Mauna Loa K coronameter) and forbidden line scans. Most of the sources in the radio maps persist for two days or more, and appear to rotate approximately with the solar rate. A coronal hole seen against the disk at all three frequencies shows interesting similarities and significant differences with the optical signatures of the hole. Elongated features of the 50 MHz corona correspond rather well to the azimuthal position of white light streamers seen in SOLWIND images. Synoptic charts made from the radio maps show overall similarities to synoptic charts constructed from (limb) coronagraph data. Some of the differences may result from the different weightings given by the radio and optical data to density and temperature, or by the different sensitivities to non-radial geometries. We show that the combined use of meter wave and optical images provide considerable new insights into the three-dimensional structure of the low to middle corona.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00152081
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  • 7
    Electronic Resource
    Electronic Resource
    Meterwave observations of a coronal hole (1987)
    Springer
    Solar physics 111 (1987), S. 419-428 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present meterwave maps showing a coronal hole at 30.9, 50.0, and 73.8 MHz using the Clark Lake Radioheliograph in October 1984. The coronal hole seen against the disk at all three frequencies shows interesting similarities to, and significant differences from its optical signatures in He i λl10830 spectroheliograms. Using the model of coronal holes by Dulk et al. (1977) we derive the electron density from the radio observations of the brightness temperature. The discrepancy between the density value derived from the Skylab EUV data and that computed from our radio data is even larger than in Dulk et al. 's comparison at similar and higher frequencies.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148529
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  • 8
    Electronic Resource
    Electronic Resource
    Coronal bright points at 6 cm wavelength (1987)
    Springer
    Solar physics 108 (1987), S. 99-111 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We report the results of the first observations of solar coronal bright points at 6 cm wavelength using the Very Large Array (VLA), with a spatial resolution of ∼ 1″.2. The maximum brightness temperature of the sources observed is ≈ 3 × 104 K with a mean value of ≈ 1 × 104 K (above the quiet Sun value). The lifetime of most sources is between 5 and 20 min. The average diameter of the sources is about 5–15″ arc. The sources are gaussian-like near the footpoint of miniature loops and they appear in groups. The observations indicate that significant fluctuations in the brightness temperature (sometimes quasi-periodic) and in the spatial extents of these sources can occur over periods of a few minutes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00152080
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  • 9
    Electronic Resource
    Electronic Resource
    6 and 20 cm wavelength observations of coronal bright points (1982)
    Springer
    Solar physics 113 (1982), S. 305-306 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00147714
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  • 10
    Electronic Resource
    Electronic Resource
    Simultaneous multi-frequency imaging observations of solar microwave bursts (1989)
    Springer
    Solar physics 121 (1989), S. 153-161 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We review the results of simultaneous two-frequency imaging observations of solar microwave bursts with the Very Large Array. Simultaneous 2 and 6 cm observations have been made of bursts which are optically thin at both frequencies, or optically thick at the lower frequency. In the latter case the source structure may differ at the two frequencies, but the two sources usually seem to be related. However, this is not always true of simultaneous 6 and 20 cm observations. The results have implications for the analysis of non-imaging radio data of solar and stellar flares.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00161693
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